Cobalt is a valuable metal used in heat-resistant alloys etc. and is recently used as a raw material for lithium ion secondary batteries, and therefore its application is expanding. Further, cobalt is mostly present in nickel ore such as nickel oxide ore in minute amounts, and is therefore obtained as cobalt metal or cobalt salts by separating nickel through the processes of dry or wet smelting and extraction.
For example, when cobalt is obtained by dry smelting, nickel ore is smelted to produce nickel matte in which nickel and cobalt are concentrated. Then, the nickel matte is subjected to leaching with mineral acid to obtain an acid solution containing nickel and cobalt, and then nickel is separated and recovered from the acid solution by a method such as solvent extraction to obtain cobalt as cobalt metal or cobalt sulfate.
Recently, wet smelting is also performed to obtain cobalt as a cobalt sulfate aqueous solution. More specifically, nickel ore is subjected to leaching with sulfuric acid under high temperature and high pressure to obtain a leachate, and a sulfating agent is added to the leachate to obtain a mixed sulfide containing nickel and cobalt. Then, the mixed sulfide is treated in the same manner as the above-described nickel matte to separate and recover nickel to obtain cobalt as a cobalt sulfate aqueous solution. Such a wet smelting method using high temperature and high pressure has the advantage that lower-grade ore can be treated as compared to the above-described dry smelting method.
When added to alloys or used as a battery material, the thus obtained cobalt is required to be high grade by separating impurities as much as possible to ensure desired performance. Particularly, in the case of the above-described wet smelting method, the impurity concentration, especially manganese concentration, of a cobalt sulfate aqueous solution obtained by separating and recovering nickel by extraction is generally required to be suppressed to a level of about several hundreds of ppm or lower in order to use the cobalt sulfate aqueous solution as a raw material.
Nickel oxide ore contains, in addition to nickel and cobalt, impurities such as manganese, magnesium, aluminum, zinc, chromium, etc. However, wet smelting has a problem with the separation of such impurities. For example, in the case of the above-described wet smelting method, manganese is leached in an acid solution together with nickel and cobalt, is distributed also to a sulfide, and shows the same behavior as cobalt also in solvent extraction performed to separate nickel and cobalt from each other, and therefore an aqueous solution containing not only cobalt but also manganese is finally obtained.
From the above fact, a cobalt sulfate aqueous solution obtained by separating and recovering nickel by wet smelting contains a considerable concentration of manganese as an impurity and is therefore difficult to add to alloys or use as a raw material for lithium ion secondary batteries. It is to be noted that in the case of dry smelting, manganese can be effectively separated as slag produced by smelting and therefore the effect of manganese on subsequent steps is small.
Various methods for removing manganese from an aqueous solution are known and examples thereof include a neutralization method, a sulfurization method, a contact filtration method, an ion exchange method, and an adsorption method. Among these methods, the neutralization method has been generally and widely used because it is an easy and reliable method for industrially treating manganese. The neutralization method is a method in which an alkaline neutralizer such as sodium hydroxide, potassium hydroxide, or calcium hydroxide is added to an aqueous solution containing manganese to adjust pH to 9 to 10 to remove manganese ions in the form of hydroxide in such an alkaline region.
However, when such a neutralization method is used to remove manganese ions in an aqueous solution in the form of hydroxide, it is difficult to separate manganese ions from, for example, metal ions precipitated as hydroxide at a pH of at most 9. Further, when manganese needs to be separated from a solution containing a high concentration of cobalt, there is a case where part of cobalt is coprecipitated when a manganese precipitate is formed by neutralization, and therefore cannot be recovered and is lost.
As a method for solving such a problem, Patent Literature 1 discloses a method in which permanganate is added as an oxidizer to water containing manganese and pH is maintained at 3 to 8 to oxidize divalent manganese ions to tetravalent manganese ions so that insoluble manganese dioxide is formed as a precipitate. However, this method has the problem of a great increase in cost because permanganate needs to be added in an amount equivalent to or more than the amount of divalent manganese ions, and in addition, a heavy metal chelator needs to be further added to remove excess permanganate.
It is to be noted that a chlorine-based oxidizer such as chlorine gas or sodium hypochlorite is inexpensive, but when such a chlorine-based oxidizer is used, there is a fear that chlorine remains in a cobalt sulfate solution. This results in chlorine contamination of cobalt sulfate crystallized out of the solution in which chlorine remains, and therefore such cobalt sulfate cannot be used in applications such as secondary battery materials required to have high purity.
On the other hand, a method is known in which nickel or cobalt is separated from a leachate by extraction. For example, Patent Literature 2 discloses a method for separating manganese contained in an acidic solution from cobalt by extraction using organophosphorus acid, carboxylic acid, and organophosphinic acid. However, when cobalt is separated by this extraction method from the above-described acidic solution obtained by subjecting nickel oxide ore to leaching with sulfuric acid, since the acidic solution also contains a high concentration of manganese, there is a problem that part of cobalt is extracted together with manganese and lost due to variations in operation conditions or part of manganese forms an oxide precipitate and the precipitate interferes with operations in the process of solvent extraction.